In a typical cellular network, also referred to as a wireless communications system or a communications system, a User Equipment (UE), communicates via a Radio Access Network (RAN) to one or more Core Networks (CNs).
A user equipment is a device that may access services offered by an operator's core network and services outside the operator's network to which the operator's radio access network and core network provide access. The user equipment may be any device, mobile or stationary, enabled to communicate over a radio channel in a communications system, for instance but not limited to e.g. mobile phone, smart phone, sensors, meters, vehicles, household appliances, medical appliances, media players, cameras, or any type of consumer electronic, for instance but not limited to television, radio, lighting arrangements, tablet computer, laptop, or PC. The user equipment may be portable, pocket storable, hand held, computer comprised or vehicle mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity.
The user equipment is enabled to communicate wirelessly in the communications system. The communication may be performed e.g. between two user equipments, between a user equipment and a regular telephone and/or between the user equipment and a server via the radio access network and possibly one or more core networks, comprised within the communications system.
The radio access network covers a geographical area which is divided into cell areas. Each cell area is served by a base station. In some radio access networks, the base station is also called Radio Base Station (RBS), evolved NodeB (eNB), NodeB, B node, Radio Network Controller (RNC), Base Station Controller (BSC) etc. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over an air interface operating on radio frequencies with the user equipment within range of the base stations.
Standardised by the third Generation Partnership Project (3GPP), High Speed Downlink Packet Access (HSPA) supports the provision of voice services in combination with mobile broadband data services. HSPA comprises High Speed Downlink Packet Access (HSDPA), High Speed Uplink Packet Access (HSUPA) and HSPA+. HSDPA allows networks based on the Universal Mobile Telecommunications System (UMTS) to have higher data transfer speeds and capacity. In HSDPA, a new transport layer channel, High Speed-Downlink Shared Channel (HS-DSCH), has been added to the UMTS release 5 and further specifications. It is implemented by introducing three new physical layer channels: High Speed-Shared Control Channel (HS-SCCH), Uplink High Speed-Dedicated Physical Control Channel (HS-DPCCH) and High Speed-Physical Downlink Shared Channel (HS-PDSCH). The HS-SCCH informs the user equipment that data will be sent on the HS-DSCH, two slots ahead. The HS-DPCCH carries acknowledgment information and a current Channel Quality Indicator (CQI) of the user equipment. The CQI is then used by the base station to calculate the amount of data to send to the user equipment in the next transmission. The HS-PDSCH is the channel mapped to the above HS-DSCH transport channel that carries actual user data. HSPA may recover fast from errors by using Hybrid Automatic Repeat reQuest (HARQ). HARQ is a technique that enables faster recovery from errors in communications systems by storing corrupted packets in the receiving device rather than discarding them. Even if retransmitted packets have errors, a good packet may be derived from the combination of bad ones.
Long Term Evolution (LTE) also defines a number of channels in the downlink, i.e. in the direction from the base station to the user equipment, for example, the Physical Downlink Shared CHannel (PDSCH) which provides user data and the Physical Downlink Control CHannel (PDCCH) which provides control information.
Multiple Input Multiple Output (MIMO) refers to any communications system with multiple antennas at the transmitter and/or the receiver, and it is used to improve communication performance. The terms input and output refer to the radio channel carrying the signal, not to the devices having antennas. At the transmitter (Tx), multiple antennas may be used to mitigate the effects of fading via transmit diversity and to increase throughput via spatial division multiple access. At the receiver (Rx), multiple antennas may be used for receiver combining which provides diversity and for combining gains. If multiple antennas are available at both the transmitter and receiver, then different data streams may be transmitted from each antenna with each data stream carrying different information but using the same frequency resources. For example, using two transmit antennas, one may transmit two separate data streams. At the receiver, multiple antennas are required to demodulate the data streams based on their spatial characteristics. In general, the minimum required number of receiver antennas is equal to the number of separate data streams. 4×4 MIMO, also referred to as four branch MIMO, may support up to four data streams. In general, MIMO may be n×n MIMO, where n is the number of antennas and is a positive integer larger than 1. For example 2×2 MIMO, 8×8 MIMO, 16×16 MIMO etc.
Several new features are added for the long term HSPA evolution in order to meet the requirements set by the International Mobile Telecommunications-Advanced (IMT-A). An objective of these new features is to increase the average spectral efficiency. Spectral efficiency is a measure of how efficiently a limited frequency spectrum is utilized. It refers to a bit rate that may be transmitted over a given bandwidth in a specific communications system. One possible technique for improving downlink spectral efficiency would be to introduce support for four branch MIMO, i.e. utilize up to four transmit and four receive antennas to enhance the spatial multiplexing gains and to offer improved beamforming capabilities. Four branch MIMO provides up to 84 Mbps per 5 MHz carrier for high Signal to Noise Ratio (SNR) user equipments and improves the coverage for low SNR user equipments.
Spatial multiplexing mentioned above is a transmission technique in MIMO to transmit independently and separately encoded data signals, so-called data streams, from each of the multiple transmit antennas. Therefore, the space dimension is reused, or multiplexed, more than one time. If the transmitter has N_t antennas and the receiver has N_r antennas, the maximum spatial multiplexing order (the number of data streams) is:N_s=min(N_t,N_r)
This means that N_s number of data streams may be transmitted in parallel, ideally leading to an N_s increase of the spectral efficiency (the number of bits per second and per Hertz (Hz) that may be transmitted over the wireless channel).
Channel feedback information, also referred to as Channel State Information (CSI), enables a scheduler to decide which user equipments that should be served in parallel. In communications systems, the CSI refers to known channel properties of a communication link. This information describes how a signal propagates from the transmitter to the receiver and represents the combined effect of, for example, scattering, fading, and power decay with distance. The CSI makes it possible to adapt transmissions to current channel conditions, which is crucial for achieving reliable communication with high data rates in multiantenna systems. The user equipment is configured to send three types of channel feedback information, i.e. CSI: CQI, Rank Indicator (RI) and Precoding Matric Indicator (PMI). CQI is an important part of channel information feedback. The CQI provides the base station with information about link adaptation parameters which the user equipment supports at the time. The CQI is utilized to determine the coding rate and modulation alphabet, as well as the number of spatially multiplexed data streams. RI is the user equipment recommendation for the number of layers, i.e. the number of data streams to be used in spatial multiplexing. RI is only reported when the user equipment operates in MIMO mode with spatial multiplexing. The RI may have the values 1 or 2 in a 2×2 MIMO configuration, i.e. one or two transmitted data streams. The RI may have the values from 1 and up to 4 in a 4×4 MIMO configuration. The RI is associated with a CQI report. This means that the CQI is calculated assuming a particular RI value. The RI typically varies more slowly than the CQI. PMI provides information about a preferred precoding matrix in a codebook based precoding. PMI is only reported when the user equipment operates in MIMO mode. The number of precoding matrices in the codebook is dependent on the number of antenna ports on the base station. For example, four antenna ports enables up to 64 matrices dependent on the RI and the user equipment capability. A Precoding Control Indicator (PCI) indicates a specific precoding vector that is applied to the transmit signal at the base station.
Introduction of four branch MIMO will require a new feedback channel structure to send the CQI and PCI information to the base station. To reduce the signalling overhead at the downlink and uplink, it was recommended to use two codewords for four branch MIMO. For designing uplink signalling channel, e.g. DPCCH or HS-DPCCH, it was agreed to use a similar structure as that of two antenna MIMO, described in 3GPP release 7. When reporting CQI, RI and PCI, the CSI may be reported in two reporting intervals. This structure is attractive in terms that it requires minimal standards change. The performance with this structure is very close to that of ideal reporting. In general, the base station needs to wait for two reporting intervals to schedule the user equipment for data transmission. If the reporting period is configured to a high value, say for example 8 msec, the base station needs to wait 16 msec to schedule the user equipment. For a high speed user equipment, this introduces delay and the performance degradation is very severe.
An overview of channel quality reporting and base station procedures for two branch (2×2) MIMO (3GPP release 7 MIMO) will now be described with reference to FIG. 1. FIG. 1 shows the messages exchanged between base station 101 and the user equipment 105 during a typical data call set up. The method comprises the following steps, which steps may be performed in any suitable order:
Step 101
From the Common Pilot Indicator CHannel (CPICH), the user equipment 105 estimates the channel and computes the CQI and the PCI. The CPICH is a downlink channel broadcast by the base station 101 with constant power and of a known bit sequence.
For two antennas, the CQI is computed as follows:
  CQI  =      {                                                      15              ×                              CQI                1                                      +                          CQI              2                        +            31                                                when            ⁢                                                  ⁢            2            ⁢                                                  ⁢            transport            ⁢                                                  ⁢            blocks            ⁢                                                  ⁢            are                                                                                                            preferred            ⁢                                                  ⁢            by            ⁢                                                  ⁢            the            ⁢                                                  ⁢            user            ⁢                                                  ⁢            equipment                                                            CQI            S                                                when            ⁢                                                  ⁢            1            ⁢                                                  ⁢            transport            ⁢                                                  ⁢            block            ⁢                                                  ⁢            is                                                                                                            preferred            ⁢                                                  ⁢            by            ⁢                                                  ⁢            the            ⁢                                                  ⁢            user            ⁢                                                  ⁢            equipment                              
Where the CQI is the channel quality per individual layer. CQI1 is the CQI of the first codeword, CQI2 is the CQI for the second codeword and CQIS is the CQI for rank 1 transmission. The number 31 is used in the equation so that 32-256 can be used for a rank 2 transmission.
It may be observed from the equation above that if the CQI is less than 31, the rank information is 1, otherwise the rank information is 2. The PCI is the precoding information bits selected in the subset of the codebook corresponding to the rank information.
Step 102
The information computed in step 101, i.e. the CQI and PCI, along with a HARQ ACK/NACK is reported to the base station 101 using e.g. the PDCCH or the HS-DPCCH.
The periodicity of HS-DPCCH is one sub-frame (e.g. 2 msec). The structure of the HS-DPCCH is shown in FIG. 2a and FIG. 2b. FIG. 2a shows an example of how the PCI and the CQI are located in the structure. As well-known, the HS-DPCCH sub-frame structure comprises one slot for HARQ ACK/NACK transmissions and two slots for CQI and PCI transmissions. In the following, even though the text or the drawings may refer to a HARQ ACK, it is appreciated that this may also be a HARQ NACK.
The HS-DPCCH sub-frame structure in FIG. 2a for the TTI=2 ms comprises a field comprising a HARQ ACK or NACK. The HARQ ACK/NACK notifies the base station 101 whether or not the user equipment 105 has received the correct downlink data. The HARQ ACK/NACK field may be defined like this: 1=NACK, 0=ACK. The CQI reflects the PCI based on CPICH strength. The each sub-frame comprises a HARQ ACK/NACK, two CQI fields and one PCI field. In other words, every sub-frame comprises the same fields.
The HS-DPCCH in 3GPP release 5 to release 9 is based on a 1×SF256 solution, where SF is short for spreading factor. The structure of the HS-DPCCH is shown in FIG. 2b. As well-known, the HS-DPCCH sub-frame structure comprises one slot for HARQ ACK/NACK transmissions and two slots for CQI and PCI transmissions. This structure may also be used for four branch MIMO.
HARQ Details: For 3GPP release 7 MIMO, the HARQ ACK/NACK codebook comprises six codewords.
CQI and PCI Details: In 3GPP release 7 there are 5 or 2×4 bits allocated for describing the CQI depending on the CQI type. There are 30 or 15 CQI values per data stream for rank 1 and rank 2, respectively, and rank is implicitly signalled via the CQI. Furthermore, the CQIs for each data stream are signalled independently of each other. In addition to CQI bits, there are two bits allocated for signalling the preferred precoding information. The 7 (or 10) information bits are then encoded into 20 channel bits that are transmitted during the second and third slot.
Returning to FIG. 1.
Step 103
Once the base station 101 receives the whole CSI report, i.e. CQI, PCI and HARQ ACK/NACK, it allocates the required channelization codes, modulation and coding, precoding control indicator to the user equipment 105 after scheduling.
Step 104
Information about the required channelization codes, modulation and coding, precoding control indicator from step 103 is transmitted to the user equipment 105 using the HS-SCCH.
Step 105
When the user equipment 105 has received the information in step 105, the user equipment 105 detects the HS-SCCH.
Step 106
Once the user equipment 105 has detected the HS-SCCH, the downlink transmission starts through a data traffic channel using the HS-PDSCH.
In general, HS-DPCCH design depends on many factors like number of codewords supported, number of HARQ processes, precoding codebook etc. Four branch MIMO should support two codewords and two HARQ processes.
The current HSDPA system (3GPP releases 7-10) supports one or two transmit antennas at the base station 101. For these systems, from channel sounding, the user equipment 105 measures the channel and reports the channel state information in one sub-frame. A sub-frame may be defined as for example one Transmission Time Interval (TTI) which may be e.g. 1 ms or 2 ms. Typically this channel state information report comprises the CQI which explicitly indicates the RI and the PCI. The user equipment sends this report periodically for every sub-frame, i.e. for every TTI to the base station. Once the base station receives this report it grants the Modulation and Coding Scheme (MCS), number of codes, rank and the PCI to each specific user equipment based on a scheduler metric. Based on this information, the base station may optimize the downlink throughput for each TTI.
The introduction of four branch MIMO will require a new feedback channel structure to send the CQI and PCI information to the base station. Since a two codeword four branch MIMO may be used, the same HS-DPCCH structure may be used for four branch MIMO as used for two branch MIMO. A problem is how to report the RI and the PCI by using the same HS-DPCCH structure as of today since the four branch MIMO requires more bits to report the RI and the PCI.